Match ends, Arsenal 2, Olympiakos 3.
Arsenal's hopes of Champions League progress are in serious jeopardy after they slumped to their second successive group stage loss at home to Olympiakos.
Arsene Wenger's side knew nothing other than victory would do after defeat by Dinamo Zagreb in Croatia in their opening game - and with two meetings with group favourites Bayern Munich still to come.
Instead it was a dreadful night for Arsenal, who must now become the first team since Galatasaray in 2012-13 to lose their first two group games and still reach the knockout stage.
The Gunners' display was summed up by the wretched first-half own goal conceded by David Ospina - preferred to Petr Cech in goal - when he dropped Kostas Fortounis's corner into his own net after Theo Walcott equalised Felipe Pardo's deflected shot.
Alexis Sanchez gave Arsenal hope with a 65th-minute header only for Alfred Finnbogason to restore Olympiakos's lead within a minute.
- Relive how the Champions League action unfolded
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Wenger's keeper calamity
When Arsene Wenger persuaded Chelsea to sell them goalkeeper Petr Cech, the theory behind his arrival was that the 33-year-old's experience, wisdom and acquaintance with success would make the difference in the big games.
So what sense did it make for Wenger to leave Cech on the bench and select his deputy Ospina for a game where Arsenal simply could not risk any mistakes or the concession of cheap goals?
Forget talk about any slight injury to Cech. He was fit enough to take his place on the bench so surely fit enough to start a game of this significance.
Fate was tempted - and could not resist as the hapless Colombian dropped a harmless corner from Fortounis into his own net just before half-time after Walcott had brought Arsenal back into it with an equaliser.
Wenger's decision was laced with high-risk - and in the context of Arsenal's hopes of making progress in the Champions League a risk not worth taking, as well as a serious error of judgement.
Will Arsenal ever learn?
Arsenal's defensive naivety was best encapsulated by the manner they went out to a very average Monaco side in the last 16 of last season's Champions League.
The Gunners pulled a goal back to make the score 2-1 in injury time in the first leg - then promptly rushed forward like lemmings in search of an equaliser, only to concede the decisive third goal that effectively put them out.
The question now is will they ever learn? Will they ever cure the defensive frailty and general vulnerability that leaves them exposed at this level?
Arsenal were victims of a deflection and a crass goalkeeping error for Olympiakos's first two goals - but the soft underbelly was on show again for the third, so easily unpicked as Finnbogason steered in the winner.
They simply did not show the maturity or composure to settle themselves. And if they cannot do that by now you have to wonder when they ever will.
Sanchez and Walcott only bright sparks
Amid a night of unrelenting gloom, the only shafts of light for Arsenal and their supporters were Walcott and Sanchez.
The Englishman continued his development as a striker with another goal, his second in successive Champions League games after scoring his 12th in 13 Premier League matches at Leicester City on Saturday.
Sanchez scored a hat-trick at Leicester and he was lively throughout, heading the second equaliser only to see Olympiakos score their third almost immediately.
In reality, this is to clutch at straws because this was a desperate Arsenal display elsewhere.
Man of the match: Alexis Sanchez
Arsenal boss Arsene Wenger: "It is still possible of course to qualify but it is difficult to swallow losing a game like that. We lost it on a lack of defensive concentration and bad luck. They had four shots on goal and we conceded three goals.
"The turning point at 2-2 is we give them a goal again straight away. If it stays 2-2 for five minutes we win the game.
"A goalkeeper can make a mistake - that doesn't explain that we lose the game. It happened to Petr Cech already this season [against West Ham]."
The stats you need to know
- Olympiakos had lost their previous 12 away games against English teams in European competition, scoring only three goals and conceding 37
- Arsenal have lost six of their last 12 Champions League home games
- The Gunners have been knocked out at the last-16 stage in the last five campaigns
- Arsenal have lost their first two Champions League matches for the first time
- The last team to qualify from the Champions League group stage after losing their first two matches was Galatasaray in 2012-13. Only nine have done it in the competition history.
- Arsenal have qualified for the Champions League second stage in the past 15 seasons
- Between 2003-04 and 2011-12, the most English defeats in a CL group stage was five, which we have already seen this season
Arsenal face the visit of Premier League leaders Manchester United on Sunday. In the Champions League their next two matches are against Bayern Munich.
- 24BellerínSubstituted forCampbellat 86'minutes
- 5GabrielBooked at 45mins
- 6KoscielnySubstituted forMertesackerat 57'minutes
- 34CoquelinSubstituted forRamseyat 60'minutes
- 11ÖzilBooked at 77mins
- 17SánchezBooked at 44mins
- 16Jimenez GagoBooked at 64mins
- 91CambiassoBooked at 76mins
- 7FortounisBooked at 77minsSubstituted forVourosat 87'minutes
- 92de Freita Couto JúniorSubstituted forSantos Fortesat 73'minutes
- 99IdeyeSubstituted forFinnbogasonat 45'minutesBooked at 75mins
- 77Santos Fortes
- Bas Nijhuis
- Shots on Target
Second Half ends, Arsenal 2, Olympiakos 3.
Attempt missed. Alex Oxlade-Chamberlain (Arsenal) right footed shot from outside the box misses to the left.
Attempt blocked. Theo Walcott (Arsenal) right footed shot from the centre of the box is blocked. Assisted by Aaron Ramsey.
Joel Campbell (Arsenal) wins a free kick on the right wing.
Foul by Leandro Salino (Olympiakos).
Joel Campbell (Arsenal) wins a free kick in the defensive half.
Foul by Alfred Finnbogason (Olympiakos).
Attempt missed. Theo Walcott (Arsenal) right footed shot from the centre of the box misses to the right. Assisted by Kieran Gibbs.
Attempt saved. Per Mertesacker (Arsenal) header from the centre of the box is saved in the bottom right corner. Assisted by Mesut Özil with a cross.
Corner, Arsenal. Conceded by Leandro Salino.
Attempt blocked. Aaron Ramsey (Arsenal) left footed shot from outside the box is blocked.
Corner, Arsenal. Conceded by Roberto.
Alexis Sánchez (Arsenal) wins a free kick on the left wing.
Foul by Felipe Pardo (Olympiakos).
Substitution, Olympiakos. Praxitelis Vouros replaces Konstantinos Fortounis.
Substitution, Arsenal. Joel Campbell replaces Héctor Bellerín.
Corner, Arsenal. Conceded by Dimitrios Siovas.
Attempt blocked. Alexis Sánchez (Arsenal) header from the centre of the box is blocked. Assisted by Kieran Gibbs with a cross.
Offside, Arsenal. Aaron Ramsey tries a through ball, but Mesut Özil is caught offside.